Until this month, movies in the Marvel Cinematic Universe have invariably ended with a taste of the following chapter in the series, the equivalent of a “Next week on … ” teaser at the end of a TV episode. But viewers who stay through the end of Avengers: Endgame won’t get a glimpse of the MCU’s future. Instead of a tacked-on scene hinting at what comes next, the credits end with a nod to where it all began. Just before the lights come up, the music dims, and we hear the clang of metal on metal: the sound of Tony Stark forging his first Iron Man suit.

That there would be no Marvel universe without Iron Man is axiomatic. Although it pales beside the amounts raked in by some of its successors, the movie’s nearly $600 million in box-office sales paved the way for what has grown to become an enterprise without cinematic precedent, a series of 22 (and counting) movies that have earned more than $18 billion combined and remade the landscape of popular culture. And there would be no Iron Man without Iron Man, which is to say without Robert Downey Jr.’s Tony Stark. He is the Marvel Cinematic Universe’s St. Peter, the rock on which the entire edifice is built.

Robert Downey Jr. is the Marvel Cinematic Universe’s St. Peter, the rock on which the entire edifice isbuilt.

Downey’s characterization has left such a mark that it’s difficult to remember a world before it. Looking back at 2008, it’s as if we were offered a choice between Downey’s devil-may-care smirk and the raked-gravel growl of Christian Bale in The Dark Knight, and while the latter won out in the short term, Downey’s mixture of insouciant self-awareness and accidental heroism has proven far more sustainable over the long haul. Bale’s Bruce Wayne felt like a costume he only reluctantly wore, a civilian identity maintained for the sole purpose of ensuring the Batcave’s rent got paid on time, but it was clear, even after he created a metallic suit that gave him the equivalent of superpowers, that Tony Stark loved being Tony Stark. Loved it as much, in fact, as Robert Downey Jr. loves being a movie star. One of the most extraordinary things I’ve ever seen was Downey walking through a crowd at the Toronto Film Festival in 2003 for the premiere of The Singing Detective, the fumbling first step on the comeback trail after years of drug abuse nearly destroyed his career. The movie, an ill-advised remake of Dennis Potter’s classic TV miniseries, was a dud, but there was energy crackling through the theater as he made his way down the aisle. You could practically see his life meter filling up as his fans tried to clap him back into existence. Some of Marvel’s stars have treated their franchise contracts like a pair of golden handcuffs, but Downey has no second thoughts about his second chance, accepting adulation as his due like the joyful Hollywood icons of yore. When Entertainment Weekly asked Downey what he wears under his Iron Man suit, he answered: “cash.”

From the first scene of Iron Man, where he’s drinking a Scotch on the rocks in the back of a military vehicle hurtling through enemy territory, Downey projected a kind of frictionless charm, tossing off lines as if he’d just come up with them himself. (In many cases, thanks to director Jon Favreau’s insistence that his actors be given time to improvise on set, he had.) Tony Stark starts off as an unapologetic cad, flirting with a female airman and bragging about bedding a year’s worth of men’s-magazine cover models, and even after a near-fatal encounter with a Stark Industries bomb forces a literal change of heart, he’s a reformed cad at best. (An early encounter in which he beds an inquisitive magazine writer and then promptly forgets her name is a reminder that casual sexism played a significant role in MCU’s initial pitch to its audience.) Just before the climactic public revelation that he is, in fact, Iron Man—an unscripted twist that was Downey’s idea—Tony tells a throng of assembled reporters that he’s “just not the hero type,” and it’s more of a confession than a misdirection. He’s not a reluctant hero, exactly—the light in Downey’s eyes the first time Iron Man’s suit takes flight makes it clear he thrills to it—but he’s better suited to the part that involves blasting bad guys than the part where he’s a paragon of virtue. (That’s Captain America’s job.) In The Avengers, he’s the fly in the superhero team’s ointment, the wrench in its works. “I’m volatile, self-obsessed, don’t play with others,” he says at their first meeting, proudly quoting the psychological profile that says he’s unfit for the group. Watching him grow into a leader—even, by the time of Captain America: Civil War, a bit of a company man—and then gain the emotional maturity to realize that other people are better suited to the task has been the MCU’s most satisfyingly detailed character arc.

Unlike most of his dewy co-stars, Downey came to the Marvel universe in his 40s, and though he’s lost none of his youthful charm or good looks, he didn’t have Chris Evans’ biceps or Chris Hemsworth’s abs. But he wore his world-weariness like a custom-tailored suit—Favreau said that Downey “always wants to be damaged by the things that happen to his character and he wants to show the damage done”—and when Tony dropped the reflexive snark, whether to whoop in delight or to look troubled or scared, you knew the moment was worth taking seriously. Endgame’s first shots of Downey, emaciated by Tony’s exile in deep space, his eyes hollowed out by grief, bring home Infinity War’s terrible losses more poignantly than a million clouds of digital dust. The movie takes Tony to new depths of despair—losing half the universe will do that to a person—and it also gives him something new to fight for: a family, whose existence may be endangered by the plan to undo Thanos’ cataclysmic snap. It’s a terrible dilemma, one that’s finally resolved only at the greatest of costs, and the moment in which Tony trades his life for the universe’s brings both the character’s story and Downey’s contract to a fitting end. People behind me audibly sobbed at his final scene, with the actor reprising the line that ad-libbed the Marvel Cinematic Universe into being, and although the Oscars seem stubbornly resistant to recognizing performances in superhero movies, you can be certain that Disney will give Downey a robust Best Actor campaign as a retirement gift (the campaign, at least, he deserves).

When Tony’s fully covered by his Iron Man suit, the Marvel movies frequently cut to a shot of Downey’s head floating in blackness, illuminated by the dim glow of a heads-up display. It would be easy for these moments to feel out of place amid the MCU’s overstuffed spectacle, but instead they’re a respite, a breath of humanity amid the computer-generated cacophony. Even in a world as sprawling as the MCU’s, sometimes a face is all you need.